This invention is based on and claims priority to Japanese Patent Applications No. 2000-3380, filed Jan. 12, 2000, the entire contents of which is hereby expressly incorporated by reference.
1. Field of the Invention
This invention relates to a valve cam mechanism for a four-cycle engine, and more particularly to an improved valve cam mechanism that decompresses a combustion chamber for easy starting of a four-cycle engine.
2. Description of Related Art
All internal combustion engines have a starting mechanism. In many applications, the starting mechanism is an electrical device that is operable with a key by the operator. This device provides a simple way to start the engine. Some simpler engines however, use a manual starting device in order to keep the engine compact. For instance, it is frequently the practice in outboard motors, and particularly those of small displacement, to incorporate a mechanism whereby the engine may be manually started. This is normally done by a rope or recoil starter that is associated with a flywheel disposed atop the crankshaft of the engine.
In order to achieve good engine performance it is a practice to use relatively high compression ratios. The use of such high compression ratios, however, gives rise to a rather large force that must be overcome by the operator to effect manual starting. There have been, therefore, proposed types of decompression mechanisms which effectively lower the compression ratio of the engine during the manual starting procedure. Preferably, such devices are automatic in nature wherein the compression ratio is lowered only long enough to facilitate starting and not long enough to interfere with the running of the engine once starting has been accomplished. That is, the decompression mechanism must be released promptly when engine is started and not work above a selected idle engine speed.
Occasionally, engines such that provided on, for example, outboard motors and lawn mowers may have camshafts extending generally vertically. One of proposed decompression mechanisms for these engines has a construction in which an actuator is mounted on a camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator may have a holder section which is arranged to hold, for example, an exhaust valve in an open position when the actuator exists in an initial position. If the engine has a rocker arm which is periodically lifted by the camshaft to actuate the exhaust valve, the holder section holds the rocker arm instead of directly holding the exhaust valve. The actuator also has a weight section that places the holder section in the initial position by the gravity, i.e., by its own weight, and moves by the centrifugal force produced by rotation of the camshaft so as to release the holder section from holding the exhaust valve in the open position.
When the holder section is in the initial position because the camshaft stands still, the exhaust valve is held in the open position in which a combustion chamber of the engine communicates with the atmosphere. Because no compression force is developed in the combustion chamber under this condition, the operator can manually start the engine. Once the engine starts, the camshaft rotates and the actuator is released from the initial position by the movement of the weight section. The combustion chamber no longer communicates with the atmosphere under this condition and normal running of the engine is thus assured. U.S. Pat. Nos. 4,453,507 and 5,150,674 disclose such decompression mechanisms in which actuators directly hold valves when starting engines.
The weight section of the actuator disclosed in U.S. Pat. No. 5,150,674, however, may cause an unexpected contact with a cam follower of the valve and if this occurs the valve may not be seated properly at the valve seat. U.S. Pat. No. 4,453,507 shows an actuator which includes a stopper section that abuts on a surface of a cam gear so as to inhibit the actuator from pivoting beyond a preset range. Because of this construction, neither the holder section nor the weight section can contact with the cam follower or the cam lobe. However, this construction may undesirably cause valve closing because the holder section can be distorted by the urging force of a valve spring that pushes the holder section when the actuator is in the initial position. If the distortion of the holder section occurs, the necessary space for the decompression, which is made between the valve and the valve seat, no longer is properly maintained. The construction disclosed in U.S. Pat. No. 5,150,674 also may have this disadvantage.
Another construction has been proposed in co-pending application Ser. No. 09/498,571 filed Feb. 4, 2000 in the name of Hiroyuki Suzuki and Kenji Yukishima and assigned to the assignee hereof. In this proposed construction, the holder section abuts on a side surface of a cam lobe of the camshaft. Distorting force, if any, exerting onto the holder section can be certainly received by the cam lobe and the necessary space can be accurately kept.
This construction, however, raises a problem that a manufacturing cost of the camshaft can increase. This is because the cam lobes are usually hardened so as to be harder than the other part of the camshaft for inhibiting wearing. Meanwhile, the side surface where the holder section abuts must be machined so that a space between the valve and the valve seat is accurately set when the holder section lifts the valve. That is, if the side surface is roughly finished, then the distance of the space can be larger or smaller than a preferred distance. This machining can be difficult because the cam lobes are hardened. A need therefore exists for an improved decompression mechanism that is less expensive to manufacture.
In accordance with one aspect of the present invention, an internal combustion engine comprises a cylinder block defining at least one cylinder bore. A piston reciprocates in the cylinder bore. A cylinder head member closes one end of the cylinder bore and defines a combustion chamber with the cylinder bore and the piston. An intake passage has an intake port through which air is introduced into the combustion chamber. An intake valve is arranged to open and close the intake port. An exhaust passage has an exhaust port through which exhaust products are discharged from the combustion chamber. An exhaust valve is arranged to open and close the exhaust port. At least one camshaft extends generally vertically. The camshaft is arranged for rotation and has cam lobes to actuate the intake valve and the exhaust valve. A decompression actuator is affixed to the camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator includes a holder section, a weight section and a stopper section. The holder section is arranged to hold the intake valve or the exhaust valve in an open position when the actuator exists in an initial position. The weight section is disposed opposite to the holder section relative to the pivot axis so as to place the holder section in the initial position. The actuator pivots when the weight section moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from the initial position. The camshaft further has a projection extending radially from the camshaft and defining a first surface on which the stopper section abuts so as to inhibit the actuator from pivoting beyond a preset range. At least the cam lobe associated with the intake valve or the exhaust valve held by the holder section includes a base circle portion and a nose portion. The base circle portion has a diameter which defines the base circle of the cam lobe. The nose portion protrudes from the base circle so as to lift the associated intake valve or the exhaust valve. The nose portion is harder than the base circle portion. The base circle portion defines a second surface projecting toward the actuator. The holder section abuts the second surface when the actuator lies in the initial position. The second surface is finished by a machining process.
In accordance with another aspect of the present invention, an internal combustion engine comprises a cylinder block defining at least one cylinder bore. A piston reciprocates in the cylinder bore. A cylinder head member closes one end of the cylinder bore and defines a combustion chamber with the cylinder bore and the piston. An intake passage has an intake port through which air is introduced into the combustion chamber. An intake valve is arranged to open and close the intake port. An exhaust passage has an exhaust port through which exhaust products are discharged from the combustion chamber. An exhaust valve is arranged to open and close the exhaust port. At least one camshaft extends generally vertically. The camshaft is arranged for rotation and has cam lobes to actuate the intake valve and the exhaust valve. A decompression actuator is affixed to the camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator includes a holder section and a weight section. The holder section is arranged to hold the intake valve or the exhaust valve in an open position when the actuator exists in an initial position. The weight section is disposed opposite to the holder section relative to the pivot axis so as to place the holder section in the initial position. The actuator pivots when the weight section moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from the initial position. The actuator and the camshaft together define means for inhibiting the actuator from pivoting beyond a preset range. At least the cam lobe associated with the intake valve or the exhaust valve held by the holder section includes a base circle portion and a nose portion. The base portion has a diameter which defines the base circle of the cam lobe. The nose portion protrudes from the base circle so as to lift the associated intake valve or the exhaust valve. The nose portion is harder than the base circle portion. The base circle portion defines a surface projecting toward the actuator. The holder section abuts on the surface when the actuator exists in the initial position. The surface is finished by a machining process.
In accordance with a further aspect of the present invention, a valve cam mechanism for a four-cycle engine having a combustion chamber comprises a valve arranged to open and close the combustion chamber to the atmosphere. A camshaft extends generally vertically. The camshaft is arranged for rotation and has a cam lobe to actuate the valve. A decompression actuator is affixed to the camshaft for pivotal movement about a pivot axis extending generally normal to an axis of the camshaft. The actuator includes a holder section and a weight section. The holder section is arranged to hold the valve in an open position when the actuator exists in an initial position. The weight section is disposed opposite to the holder section relative to the pivot axis so as to place the holder section in the initial position. The actuator pivots when the weight section moves by centrifugal force produced by the rotation of the camshaft so as to release the holder section from the initial position. The actuator and the camshaft together define means for inhibiting the actuator from pivoting beyond a preset range. The cam lobe includes a base circle portion and a nose portion. The base circle portion has a diameter which defines the base circle of the cam lobe. The nose portion protrudes from the base circle so as to lift the associated valves. The nose portion is cured so as to be harder than the base circle portion. The base circle portion defines a surface projecting toward the actuator. The holder section abuts on the surface when the actuator exists in the initial position. The surface is finished by a machining process.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiment which follows.